The present invention relates to an illumination device, and more particularly to a laser illumination device based on electrically switchable Bragg gratings that reduces laser speckle.
Miniature solid-state lasers are finding their way into a range of display applications. The competitive advantage of lasers results from increased lifetime, lower cost, higher brightness and improved colour gamut. Although lasers offer much more compact illumination solutions than can be provided with conventional sources such as LED the demand for yet more compressed form factors continues unabated. Classical illumination designs using beam splitters and combiners fail to meet the requirements. It is known that delivering laser illumination via waveguide optics can result in 50% reduction in size compared with conventional lens combiner splitter schemes.
Laser displays suffer from speckle, a sparkly or granular structure seen in uniformly illuminated rough surfaces. Speckle arises from the high spatial and temporal coherence of lasers. Speckle reduces image sharpness and is distracting to the viewer. Several approaches for reducing speckle contrast have been proposed based on spatial and temporal decorrelation of speckle patterns. More precisely, speckle reduction is based on averaging multiple sets of speckle patterns from a speckle surface resolution cell with the averaging taking place over the human eye integration time. Speckle may be characterized by the parameter speckle contrast which is defined as the ratio of the standard deviation of the speckle intensity to the mean speckle intensity. Temporally varying the phase pattern faster than the eye temporal resolution destroys the light spatial coherence, thereby reducing the speckle contrast. Traditionally, the simplest way to reduce speckle has been to use a rotating diffuser to direct incident light into randomly distributed ray directions. The effect is to produce a multiplicity of speckle patterns while maintaining a uniform a time-averaged intensity profile. This type of approach is often referred to as angle diversity. Another approach known as polarization diversity relies on averaging phase shifted speckle patterns. In practice neither approach succeeds in eliminating speckle entirely.
It is known that speckle may be reduced by using an electro optic device to generate variations in the refractive index profile of material such that the phase fronts of light incident on the device are modulated in phase and or amplitude. U.S. Pat. No. 8,224,133 with issue date 17 Jul. 2012 entitled LASER ILLUMINATION DEVICE discloses a despeckler based on a new type of diffractive electro optical device known as an electrically Switchable Bragg Grating (SBG).
There is a need for a compact laser illuminator that efficiently combines waveguide optics with a diffractive electro-optical despeckler.